With the development in digital technologies in recent years, electronic devices such as mobile information equipment and information home appliances have higher functionality. Thus, demands for increasing the capacity of nonvolatile storage elements included in these devices, reducing the write power, accelerating writing and reading operations, and increasing the life span of these devices have been increasing.
To meet such demands, attention is focused on a new variable resistance nonvolatile storage element (also referred to as “variable resistance element”) including a variable resistance layer as a material of a storage unit. Since the variable resistance nonvolatile storage element has such a simple structure and simply performs operations, it is expected that the nonvolatile storage element can further be miniaturized and the cost can be reduced. Since the resistance state of the variable resistance nonvolatile storage element sometimes changes between the low resistance state and the high resistance state by orders of magnitude not longer than 100 nanoseconds (ns), the attention is further focused on the variable resistance nonvolatile storage elements in view of its higher operating speed.
Such variable resistance nonvolatile storage elements comprising metal oxides can be largely divided into two types, depending on a material to be used in each of the variable resistance layers.
One type is the variable resistance nonvolatile storage elements comprising perovskite materials in the variable resistance layers, as disclosed in PTL 1 and others. The other is the variable resistance nonvolatile storage elements that are compounds comprising only transition metals and oxygen, using binary transition metal oxides.
In the binary transition metal oxides, controlling the compositions when manufactured and forming the films are relatively easy. Furthermore, the binary transition metal oxides have relatively favorable compatibility with semiconductor manufacturing processes. For example, PTL 2 discloses variable resistance elements comprising, as variable resistance materials, (i) transition metal oxides of stoichiometric composition, such as nickel (Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt (Co), iron (Fe), copper (Cu), and chrome (Cr), and (ii) oxides whose composition is deficient in oxygen compared to its stoichiometric composition (hereinafter referred to as oxygen-deficient oxides). Furthermore, PTL 3 discloses a nonvolatile storage element comprising an oxygen-deficient tantalum (Ta) oxide as a variable resistance material.
Furthermore, it is also reported that a variable resistance nonvolatile storage element has two different operation modes, namely, unipolar (monopolar) switching and bipolar switching.
The unipolar switching is an operation mode in which a resistance value changes with application of electric pulses having the same polarity and different amplitudes between a lower electrode and an upper electrode of a variable resistance nonvolatile storage element, which is disclosed by PTL 2 and others. In contrast, the bipolar switching is an operation mode in which a resistance value changes with application of electric pulses of positive and negative polarities between a lower electrode and an upper electrode of a variable resistance nonvolatile storage element, which is disclosed by PTLs 1 and 2.